Strontium titanate-ferrate thermistor compositions



United States Patent Office US. Cl. 252-521 4 Claims ABSTRACT OF THEDISCLOSURE Novel strontium titanate ferrate compositions of the formulaSrTi Fe 1p in which x has a value of from 0.01 to 0.999 and 5 representsan oxygen deficiency, have a negative temperature coefiicient ofelectrical resistance and are useful as resistance heating bars;-particularly at high temperatures.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to compositions for use as thermistors having a negativetemperature coefiicient of electrical resistance. For purposes of thisinvention, the word thermistor may be defined as a thermally sensitiveelectrical resistor. Thermistors find wide-spread applications as highlysensitive thermostats and may be used in many applications for automaticcontrols. The novel compositions are novel thermistor materials which,over a wide range of temperatures, exhibit negative coefficients ofelectrical resistance. These materials make possible greater controlover the shape of the temperature-resistance curve than has previouslybeen found possible with many known compositions.

The compositions of this invention are new, sintered strontiumtitanate-base thermistors containing ferric oxide as dopant.

Description of the prior art The host crystal for the compositions ofthis invention is strontium titanate of the chemical formula SrTiOUndoped SrTiO, constitutes an undistorted cubic perovskite, with the THoctahedrally surrounded by 6 oxygen atoms. It has been proposed tomodify perovskite by substituting tetravalent iron or manganese forTi+*see Cesare Brisi, Annali di Chimica 45 431-8 (1955). Suchtetravalent iron substitution does not, of course, create crystallattice vacancies such as characterize the products of the presentinvention in which trivalent ions are substituted.

SUMMARY .Now according to the present invention it has been found thatthe solubility range of Fe in the perovskite structure is extreme, sinceessentially total substitution is possible with the end member of theseries, SrFeOas being a strontium ferrate III, which is a defectstructure of the established strontium ferrate (IV) SrFeO In the casewhere Fe O is substituted in the strontium titanate host crystal,electroneutrality after substitutional exchange of TH by Fe' is achievedby way of anion defects as represented in the formula SrTi+ ,,Fe

(3-0.5x)0.5x representing the lattice vacancies). Electrically, the

3,511,786 Patented May 12, 1970 representation of the vacancies issometimes omitted for convenience, but it will be understood that aniondefects in the crystal lattices are present as called for by theformula: SrTi Fe O BRIEF DESCRIPTION OF THE DRAWING In the drawings:

FIG. 1 is a graph of resistivity plotted against temperature for thecomposition of the invention having the fmmllla aos o.ss a.saso.4'zs andI FIG. 2 is a graph of resistance plotted against temperature for thecomposition of the invention,

(dashed line) as compared with solid SiC (solid line), a commercialresistor material, and

FIG. 3 is a graph of resistivity plotted against co position for thesystem of the invention (1x) x (3-0.5x)0.5x DESCRIPTION OF THE PREFERREDEMBODIMENTS To illustrate the preparation of particular compositions ofthis invention and to show the properties of these compositions, thefollowing examples are given:

EXAMPLE 1 This example will illustrate preparation of the compositionSrTi Fe 0 This composition was prepared by weighing stoichiometricquantities of the reactant materials according to the formulae andequation as follows:

To make the product of this equation, 4.000 grams SrCO 0.1082 gram TiOand 2.0552 grams Fe O all of the best commercially available purity andgenerally 99.9%.|-, were weighed to the nearest $4 milligram, and thesepowders were dry blended and fired in a Pt dish to 900 C. for 10 to14'hours. The product of this reaction was 7 measured.

For measurement of electrical properties, the ends of the bars weresilvered and leads were attached to thesilvered portions. A two-pointprobe was used to measure the resistivity, and results for thecomposition of this example were as follows:

at 30 C.: 2.4 x10t2cm. at -196 C.: 1.6 x10 0cm. 1 at 850 C.: 2.0x 10Gem.

EXAMPLE 2 Using the procedure of Example 1, a composition of.

the chemical formula SrTi Fe O was prepared. For this composition thefollowing weights of reactants were used: 4.0000 grams SrCO 0.2165 gramTiO and 1.9468 grams Fe O These reactants were fired, ground forhomogenization, compacted, and refired as described in Example 1.

The electrical properties of the product of this example are as follows:

at room temperature: 1.2)(10 (2cm. at -196 C.: 6.9)(10 9cm.

made in the strontium titanate crystal in which 2: in the formula SrTiFe O had a value from 0.01 to 0.05. The structural data for thesecompositions, including the compositions of Examples 1, 2, and 3 were asfollows:

TABLE 11 [X-ray crystallographic data for compositions Sr'Ii Fe,.0 wherex=0.01 to 0.95]

X-ray Value a Moi. vol. density Ex. No. Composition of x (A=l=0-001) A(calm) 1. SrTiu,o F0n.9s02.sza- O. 95 3. 875 58. 18 5. 24 2. SrTl F 0.90 3-. 879 58. 36 5. 22 I MJFGMnOz r0 0. 60 3. 892 58. 95 5. 17 4. SrTl0.01 3.905 60. 92 5. 11 5 SrTl 0. 3. 903 59. 46 5.12 6 SrTlo. F 0. 3.901 59. 36 5. 13 7.. BlTlo.5uF o 500 O. 50 3. 896 59. 14 5. 15 8 SrTlmwF0. 70 3. 888 58. 77 5. l8

X-ray analysis of this product showed it to be a singlephase compositionwith a density of 5.2 grams per ml.- and a cell edge of 1879:0001 A.

A composition of this example was operated as a resistance heating barup to a temperature of 1200' C. in air. The power measurements for thisheating bar are recorded in Table I.

Using the procedure of Example 1, a composition of the formula SrTi Fe Owas prepared. The amounts oi reactants used for the preparation of thiscomposition were as follows: 6.1594 grams SrCO 2.0000 grams TiO and1.3324 grams Fe O As in Example 1, the bars of this composition wereprepared by the steps of blending, heating, grinding, compacting andrefiring. There is shown in FIG. 2 a graph for resistance plottedagainst temperature for the composition of this example as compared withsolid SiC, a commercial thermistor material. It will be seen from thisgraph that the composition of this invention shows an 'even strongertemperature dependence than does the commercially available and widelyused SiC material of the prior art.

' In the manner of the above examples, other compositions were preparedin which substitutions of l e- 0 were There is shown in FIG. 3 a plot ofresistivity in ohms versus increasing values for x as in the abovecompositions.

From the results which are given above it is seen that there have beenprepared compositions of the general formula SrTi Fe O which are usefulas negative temperature thermistors, and for use, particularly at hightemperatures, as resistance heating bars. These compositions will befound particularly useful as ingredients in ceramic compositions forresistance heating elements, as for example, ceramic hot plates.

I claim:

1. A strontium titanate ferrate composition having the formula SrTi Fe Oin which x has a value of from 0.01 to 0.999 and 15 represents an oxygendeficiency, said composition being characterized by having a negativetemperature coeflicient of resistance.

2. A composition of claim 1 having the formula:

o.os o.s5 2.525o.415 3. A composition of claim 1 having the formula:

o.1 o.9 2.55o.45 4. A composition of claim 1 having the formula:

o.4 o.6 2.'1o.a

References Cited UNITED STATES PATENTS 3,105,800 10/ 1963 Watenabe252-520 3,420,776 1/ 1969 Hepplewhite et al. 252-520 3,441,517 4/1969Brauer et al. 252-520 ,DOUGLAS J. DRUMMOND, Primary Examiner US. Cl.X-R.

